The superoxide anion (O2−) is a potentially harmful cellular by-product produced primarily by errors of oxidative phosphorylation in mitochondria (Cleveland and Liu, Nat. Med., 2000, 6, 1320-1321). Some of the targets for oxidation by superoxide in biological systems include the iron-sulfur dehydratases, aconitase and fumarases. Release of Fe (II) from these superoxide-inactivated enzymes results in Fenton-type production of hydroxyl radicals which are capable of attacking virtually any cellular target, most notably DNA (Fridovich, Annu. Rev. Biochem., 1995, 64, 97-112).
The enzymes known as the superoxide dismutases (SODs) provide defense against oxidative damage of biomolecules by catalyzing the dismutation of superoxide to hydrogen peroxide (H2O2) (Fridovich, Annu. Rev. Biochem., 1995, 64, 97-112). Two major classes of superoxide dismutases exist. One consists of a group of enzymes with active sites containing copper and zinc while the other class has either manganese or iron at the active site (Fridovich, Annu. Rev. Biochem., 1995, 64, 97-112).
The soluble superoxide dismutase 1 enzyme (also known as SOD1 and Cu/Zn superoxide dismutase) contains a zinc- and copper-type active site (Fridovich, Annu. Rev. Biochem., 1995, 64, 97-112). Lee et al. reported the molecular cloning and high-level expression of human cytoplasmic superoxide dismutase gene in E. coli in 1990 (Lee et al., Misaengmul Hakhoechi, 1990, 28, 91-97). Studies of transgenic mice carrying a mutant human superoxide dismutase 1 gene, for example, transgenic mice expressing a human SOD1 gene bearing glycine 93 to alanine (G93A) mutation.
Mutations in the superoxide dismutase 1 gene are associated with a dominantly-inherited form of amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's disease) a disorder characterized by a selective degeneration of upper and lower motor neurons (Cleveland and Liu, Nat. Med., 2000, 6, 1320-1321). The deleterious effects of various mutations on superoxide dismutase 1 are most likely mediated through a gain of toxic function rather than a loss of superoxide dismutase 1 activity, as the complete absence of superoxide dismutase 1 in mice neither diminishes life nor provokes overt disease (Al-Chalabi and Leigh, Curr. Opin. Neurol., 2000, 13, 397-405; Alisky and Davidson, Hum. Gene Ther., 2000, 11, 2315-2329).
Cleveland and Liu proposed two models for mutant superoxide dismutase 1 toxicity (Cleveland and Liu, Nat. Med., 2000, 6, 1320-1321). The “oxidative hypothesis” ascribes toxicity to binding of aberrant substrates such as peroxynitrite or hydrogen peroxide which gain access to the catalytic copper ion through mutation-dependent loosening of the native superoxide dismutase 1 protein conformation (Cleveland and Liu, Nat. Med., 2000, 6, 1320-1321). A second possible mechanism for mutant superoxide dismutase 1 toxicity involves the misfolding and aggregation of mutant superoxide dismutase 1 proteins (Cleveland and Liu, Nat. Med., 2000, 6, 1320-1321). The idea that aggregates contribute to ALS has received major support from the observation that murine models of superoxide dismutase 1 mutant-mediated disease feature prominent intracellular inclusions in motor neurons and, in some cases, in the astrocytes surrounding them as well (Bruijn et al., Science, 1998, 281, 1851-1854). Furthermore, Bruijn et al. also demonstrate that neither elimination nor elevation of wild-type superoxide dismutase 1 was found to affect disease induced by mutant superoxide dismutase 1 in mice (Bruijn et al., Science, 1998, 281, 1851-1854).
Riluzole, a glutamate regulatory drug, is approved for use in ALS patients in some countries, but has only a modest effect on survival. Accordingly, there remains an unmet need for therapeutic regimens that slow familial ALS disease progression and increase survival of familial ALS patients.